1. Field of the Invention
The present invention relates to polyolefin foam beads which can be processed over a wide temperature range to give molded articles.
2. Description of the Related Art
The production of molded articles from polyolefin foams is known. Accordingly, a closed but vented mold is filled with foam beads, which may be loose or under pressure, and these are intimately fused by applying steam.
The shape of the molded articles can vary greatly. In industry, molded articles exhibit very large differences in wall thicknesses. For example, areas of 20 cm thickness contrast with thin fillets or tapered zones having a wall thickness of only a few millimeters.
The application of steam must be sufficiently intensive to bring about complete fusion of the foam beads with one another in the interior of the thick areas. This intensive application of steam causes over stressing of the foam beads in the thin-walled areas as a result of long exposure to excessively high temperatures.
Damage to the foam beads has been observed under these conditions. The base material overblows or begins to melt, and the cell structure is partially or completely destroyed. The foam beads collapse in these areas after the molded articles have been cooled.
If, on the other hand, the application of steam is gentle, there is insufficient fusion in the interior of the thick areas. In addition, the surfaces have interstices and are matt.
There is, furthermore, increasing demand in the market for molded articles having a very smooth surface, extending to compaction of the outer layer of material, the bead structure being no longer recognizable. A marked improvement of this type in the quality of the surface can be achieved if increased steam pressures are used with lengthened steam application time. This, however, generally likewise causes damage to the foam beads.
An object of he present invention is to prepare, from a polyolefinic material, foam beads which can be processed without damage in a wide range of processing conditions.